The Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) which is a set of enhancements to the Universal Mobile Telecommunications System (UMTS) is introduced as 3GPP Release 8. The 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) for a downlink, and uses single carrier frequency division multiple access (SC-FDMA) for an uplink, and adopts multiple input multiple output (MIMO) with up to four antennas. In recent years, there is an ongoing discussion on 3GPP LTE-Advanced (LTE-A), which is a major enhancement to the 3GPP LTE.
The commercialization of the 3GPP LTE (A) system is being recently accelerated. The LTE systems are spread more quickly as respond to users' demand for services that may support higher quality and higher capacity while ensuring mobility, as well as voice services. The LTE system provides for low transmission delay, high transmission rate and system capacity, and enhanced coverage.
To increase the capacity for the users' demand of services, increasing the bandwidth may be essential, a carrier aggregation (CA) technology or resource aggregation over intra-node carriers or inter-node carriers aiming at obtaining an effect, as if a logically wider band is used, by grouping a plurality of physically non-continuous bands in a frequency domain has been developed to effectively use fragmented small bands. Individual unit carriers grouped by carrier aggregation is known as a component carrier (CC). For inter-node resource aggregation, for each node, carrier group (CG) can be established where one CG can have multiple CCs. Each CC is defined by a single bandwidth and a center frequency.
Recently, a wireless access network configuration has been changed such that various types of small cells having small sizes, such as a pico cell, a femto cell, etc., interact with a macro cell having a relatively large size. The wireless access network configuration aims to provide a high data rate to final UEs and thus increase Quality of Experience (QoE) for the final UEs in a situation where multi-layer cells co-exist in a hierarchical structure basically involving a macro cell.
According to one of the current 3rd Generation Partnership Project (3GPP) standardization categories, Small Cell Enhancements for E-UTRA and E-UTRAN SI; e.g., RP-122033, enhancement of indoor/outdoor scenarios using low-power nodes is discussed under the title of small cell enhancement. In addition, scenarios and requirements for the small cell enhancement are described in 3GPP TR 36.932.
Meanwhile, the usage of small cell is getting grown in many fields nowadays, such as pico cells, small cells under dual connectivity, etc. To properly perform communication between the small cells and UEs, improvements related to conventional control signals, such as reference signals and synchronous signals, have been discussed